A barium titanate (BaTiO3)-based semiconductor ceramic generates heat upon the application of a voltage and has PTC characteristics in which the resistance value rapidly increases when the temperature exceeds the Curie temperature Tc at which a phase transition takes place from a tetragonal crystal to a cubic crystal.
In such a semiconductor ceramic having PTC characteristics, the resistance value increases when the temperature exceeds the Curie temperature Tc because of the generation of heat caused by the application of a voltage. As a result, a current does not easily flow and the temperature decreases. When the temperature is decreased and thus the resistance value is decreased, a current easily flows again and the temperature is increased. By repeating the above-described process, the temperature or current is caused to converge to a certain temperature or current in such a semiconductor ceramic. Therefore, such a semiconductor ceramic is widely used as a thermistor for a heater or a motor starting device.
Since a PTC thermistor used for a heater or the like is used at high temperature, the Curie temperature Tc is required to be high. Therefore, the Curie temperature Tc has been conventionally increased by replacing part of Ba of BaTiO3 with Pb.
However, Pb is an environmentally unfriendly substance and thus, in consideration of environment, the development of a lead-free semiconductor ceramic that substantially does not contain Pb has been demanded.
Patent Document 1 discloses a method for producing a BaTiO3-based semiconductor ceramic, the method including adding at least one of Nb, Ta, and a rare-earth element to a structure of Ba1-2X(BiNa)XTiO3 (where 0<x≦0.15) obtained by replacing part of Ba of BaTiO3 with Bi—Na, sintering the structure in a nitrogen atmosphere, and performing a heat treatment in an oxidizing atmosphere.
In Patent Document 1, there is provided a lead-free BaTiO3-based semiconductor ceramic having a high Curie temperature Tc of 140 to 255° C. and a temperature coefficient of resistance of 16 to 20%/° C.
Patent Document 2 discloses a semiconductor ceramic composition whose composition formula is represented by [(A10.5A20.5)x(Ba1-yQy)1-x]TiO3 (where A1 is at least one of Na, K, and Li, A2 is Bi, and Q is at least one of La, Dy, Eu, and Gd), wherein the x and y satisfy 0<x≦0.2 and 0.002≦y≦0.01.
Also in Patent Document 2, there is provided a lead-free semiconductor ceramic composition having a Curie temperature Tc of 130° C. or more.    Patent Document 1—Japanese Unexamined Patent Application Publication No. 56-169301    Patent Document 2—Japanese Unexamined Patent Application Publication No. 2005-255493